In recent years, certain highly branched polymers have attracted considerable attention because of their special physical and chemical properties. Hyperbranched polymers are polymers having a very high degree of branching and with at least some branches coming off of other branches. A dendrimer is an ideal type of hyperbranched polymer having a perfectly regular network of generational branching of branches from other branches. Dendritic polymers are hyperbranched polymers that possess a high degree of regularity in a branching network, even if not attaining the perfect regularity of true dendrimers.
One useful property of many highly branched polymers, and especially dendritic polymers, is that they tend to exhibit significantly different flow properties than polymers without significant branching. For linear polymers, polymer viscosity is generally a function of the polymer chain length, and thus, also the polymer molecular weight, with viscosity generally increasing with increasing molecular weight. Dendritic polymers, however, tend to have significantly lower viscosities than linear polymers of similar overall composition and molecular weight. Because of the reduced viscosity of dendritic polymers, they have been proposed as viscosity-modifying additives. For example, dendritic polymers may be mixed with other polymers to form compositions with reduced viscosity for advantageous melt processing, such as is encountered during processes for foaming, molding, and extruding polymeric materials. Because the polymer mixture including the dendritic additive has a lower melt viscosity than would be the case in the absence of the dendritic additive, melt processing can be accomplished with lower energy requirements.
As another example of viscosity modification, highly branched star-shaped polymers have been proposed as viscosity modifiers in motor oil. Star-shaped polymers have branching arms (typically four branching arms) that radiate from a central core. At low temperatures the arms are tightly folded about the core and do not have a large effect on the viscosity of the mixture. At higher temperatures, however, the arms unfold to provide a significant viscosity increasing effect for the mixture at the higher temperatures.
Another significant property of highly branched polymers is the large number of sites available for functionalization at the free terminal ends of the numerous exterior branches. This feature of dendritic polymers provides great promise for advantageous functionalization of dendritic polymers for various uses, such as, for example, as ion exchange resins, ligands for chelation or complexation, polyelectrolytes, uni-molecular micelles, and solubility modifying agents. For these and other applications, a highly dendritic structure and a large molecular weight would typically be desirable.
Even with these advantageous features, however, dendritic polymers have been slow to gain commercial acceptance. This is largely because methods for synthesizing highly dendritic polymers tend to be very cumbersome and expensive. Also, the dendritic polymers that are produced often do not have a sufficiently high molecular weight or a sufficiently large number of generations of branching, or both, for advantageous use in many applications. Furthermore, only a limited number of different polymer compositions have been manufactured in a highly dendritic form.
Two basic approaches have been used to synthesize highly branched polymers. One approach is divergent. Divergent synthesis involves building the molecule in an outward fashion beginning at a central core, or apex. The structure grows by adding branches, one generation at a time, to the exterior portions of the growing structure. Some divergent syntheses have been reported to yield highly dendritic structures, having narrow polydispersities, or variations in size and structure. One problem with divergent synthesis, however, is that the technique is complex, and generally requires intermediate isolation and purification between generations. Divergent synthesis is, therefore, practically limited to manufacture of dendritic structures containing only a small number of generations of branching. Furthermore, product yields are often low because of the numerous isolation and purification steps that may be required.
A second approach to synthesizing highly branched polymers is convergent. Convergent synthesis involves building the molecule in an inward fashion, beginning with what will be the exterior branches for the final structure. During convergent synthesis, the dendritic structure grows through successive coupling reactions to form increasingly larger branched structures. The final coupling reaction creates the final focal point (also referred to as the apex or core) of the final dendritic structure. Some highly dendritic structures have been reported to have been manufactured by convergent synthesis. Similar to the situation with divergent synthesis, however, convergent methods reported to have produced highly dendritic structures may involve isolation and purification between generations, and may also suffer from resulting low yields. This is particularly a problem when attempting to make a highly dendritic structure with a large number of generations of branching or with a high molecular weight. As with divergent synthesis, convergent synthesis has practically been limited to making dendritic structures containing only a small number of generations.
Furthermore, only a very limited number of polymer compositions have been demonstrated as manufacturable by current synthesis techniques, and these compositions often are made with only a small number of generations of branching and/or with a relatively low molecular weight. Attempts to make dendritic compositions with more generations of branching and/or higher molecular weights have often either become too complex and expensive, or resulted in materials that failed to achieve a highly dendritic structure. There has been little reported success in making dendritic structures of vinyl polymers, such as polystyrenes, and especially for making such polymers having a reasonably high molecular weight, a large number of generations of branching and a highly dendritic structure and a narrow polydispersity.
There is a need for synthesis techniques for making dendritic polymers that are less complex and less expensive than current techniques. There is also a need for synthesis techniques better suited for manufacture of dendritic polymers with a large number of generations of branching and high molecular weights. Furthermore, there is a need for dendritic polymers of a greater variety of compositions, and especially for dendritic vinyl polymers of a high molecular weight and narrow polydispersity.